1. Field of the Invention
The present invention relates to a method for detecting tie-ups in an automatic transmission for a vehicle.
2. Background Art
Vehicles today increasingly employ the use of adaptive learning systems to improve a wide variety of vehicle operations. For example, various engine characteristics may be monitored such that actual outputs are compared to desired outputs, so that adjustments can be made to bring the two closer together. Such engine characteristics may include those that affect performance, fuel efficiency, or some other characteristic to be controlled. Other systems within a vehicle may be similarly monitored, with adjustments being made to bring the actual system outputs closer to the desired system output. Again, such outputs may affect performance or efficiency, or may be directly related to occupant comfort.
An example of the latter is found in a vehicle transmission, which not only affects vehicle performance and efficiency, but also has a direct impact on the quality of the ride. This is particularly noticeable during times when the transmission is shifting gears. A smooth transition between gears may be largely imperceptible to vehicle occupants; conversely, a rough transition between gears can contribute to occupant discomfort, and therefore, has a negative effect on the quality of the ride. One reason a transmission may not shift smoothly, is because tie-ups can occur. When a transmission shifts smoothly from one gear to the next, bands or clutches are engaged on the new gear just as the bands or clutches are disengaged on the previous gear. If, however, there is even a slight error in the timing between the engagement and disengagement of the respective bands or clutches, both gears may be held simultaneously. This phenomenon is known as a tie-up.
One device for determining when a tie-up occurs is described in U.S. Pat. No. 6,106,435 issued to Endo et al. on Aug. 22, 2000. The Endo et al. device decides whether a transmission tie-up has occurred, and in particular, decides the degree of tie-up. The degree of tie-up is determined from an equation that requires integration of the difference between instantaneous and average transmission output shaft acceleration. Alternatively, the Endo et al. device may determine the degree of tie-up based on the maximum amplitude of the fluctuation of the transmission output shaft speed. Endo et al. also briefly mentions prohibiting the calculation of the degree of tie-up in the case of a “bad road surface”.
One limitation of Endo et al. is that the parameters used to determine the degree of tie-up involve the integral of an acceleration term—i.e., a speed term—which may not be as sensitive to output shaft changes as an acceleration term, or the derivative of an acceleration term. Thus, as time derivatives of the speed of the transmission output shaft are taken, the resulting parameters are increasingly sensitive to transmission output shaft changes. In addition, although there may be many ways to determine whether a road surface is “bad”, Endo et al. does not describe a specific device or method for determining a bad road surface.
Therefore, a need still exists for a method to detect transmission tie-ups which utilizes a sensitive parameter, such as the time based derivative of a transmission shaft acceleration, and also provides a specific method to determine a false tie-up, such as might be indicated on a bad road surface.